An accelerometer is one of the major sensors used in navigational systems, particularly inertial navigational systems, and on-board automotive safety control systems. Automotive examples of accelerometer use include various anti-lock braking systems, active suspension systems, and seat belt lock-up systems.
More generally, an accelerometer is a device which measures acceleration and, in particular, an accelerometer measures the force that is exerted when a moving body changes velocity. The moving body possesses inertia, which tends to resist the change in velocity. It is this resistance to sudden change in velocity that is the origin of the force which is exerted by the moving body, and which is proportional to the acceleration component in the direction of the movement, when the moving body is accelerated.
Conventional manufacturing techniques for metal and insulation material are limited in their ability to produce mechanical assemblies that are smaller, or lower in cost, than the devices currently available. Thus, the prior art accelerometers are constrained by the materials utilized therein and the methods of fabrication thereof rather than by problems of mechanical or electrical design. It is thus desirable to provide a new technology for fabrication of accelerometers in order to reduce the size and cost thereof.
More particularly, it is desired to reduce the size of accelerometer devices in order to reduce the length of thermal and electrical conduction paths used therein. It is known that long paths of conduction may result in large thermal gradients. These effects reduce the accuracy, stability and performance of the accelerometer. Further, the large size of currently available accelerometers results in increased sensitivity to stray capacitance and electromagnetic radiation. Accordingly, the presently available accelerometers suffer from disadvantages caused by the large size thereof, including particularly limitations on stability and accuracy. Yet another disadvantage resulting from the large size of presently available accelerometers is a low resonant frequency of the proof mass caused by the increased size, which increases the response time of the accelerometer.
Accordingly, while prior art inertial accelerometers may have dynamic ranges and sensitivities which are in the neighborhood of one part in 10.sup.7, the performance of the accelerometer and its reliability may be improved by an order of magnitude upon a reduction in size.
Some accelerometers, as disclosed in U.S. Pat. Nos. 3,742,767 and 4,393,710 to Bernard, suspend the free-mass in its equilibrium position with electromagnetic forces from electrodes supported by a surrounding cage. Since the free-mass is not constrained by any physical attachment to the housing of the accelerometer, it can indicate acceleration in three coordinate systems. However, such accelerometers cannot be fabricated as integral units through micromachining techniques due to the difficulty of satisfactorily forming electrodes on the vertical walls of the accelerometer.
Microaccelerometers like the ones described in U.S. Pat. Nos. 4,922,756 and 4,932,261 to Henrion, F. Rudolph, A. Jornod and P. Bencze, Silicon Microaccelerometers, Transducers '87, Rec. of the 4th Int'l Conference on Solid-State Sensors and Actuators, 1987, 395-398, and D. Satchell and J. Greenwood, Silicon Microengineering for Accelerometers, Rec. of the Int. Conf. on the Mech. Technol. of Inertial Devices, 191-193, 1987, use a spring suspension system to position the proof-mass in an equilibrium position. When inertial forces displace the proof-mass from the equilibrium position, spring tension returns the proof-mass to its equilibrium position.
Another type of accelerometer, as described in U.S. Pat. No. 4,945,765 to Rosxhart, U.S. Pat. No. 4,901,570 to Chang, U.S. Pat. No. 4,893,509 to MacIver, U.S. Pat. No. 4,736,629 to Cole, U.S. Pat. No. 4,706,374 to Murakami, P. Chen, et al. Integrated Silicon Microbeam PI-FET Accelerometer, IEEE Trans. Electron Devices, Vol. ED-29, no. 1, 27-33, January 1982, M. Motamedi, Acoustic Accelerometers, IEEE Trans. Ultrason. Ferro Elec. Freq. Contr. Vol., UFFC-34, no. 2, 237-242, March 1987, K. Petersen, Silicon as a Mechanical. Material, Proc. IEEE, Vol. 70, no. 5, 420-457, May 1982, L. Roylance, A Batch-Fabricated Silicon Microaccelerometer, IEEE Trans. Electron Devices, Vol. ED-26, no. 12, 1911-1917, December 1979, and R. Muller, Heat and Strain Sensitive Thin-Film Transducers, Sensors and Actuators, Vol. 4, 173-182, December 1983, employs a cantilever system to maintain the proof-mass in an equilibrium position and return it to that position after its displacement during acceleration.
Although accelerometers utilizing the spring and cantilever systems can be constructed to a microelectronic size by micromachining techniques, movement of their proof-masses is constrained in at least one of three dimensions. Further, such systems lack the sensitivity and linearity achieved with free-floating masses. In view of these disadvantages of current microaccelerometers, a need exists for new designs which can be fabricated by micromachining techniques, yet have free-floating masses which generate sensitive readings.